Quaternary ammonium hydroxides such as tetramethylammonium hydroxide (TMAH) and tetraethyl ammonium hydroxide (TEAH) are strong organic bases that have been known for many years. Such quaternary ammonium hydroxides have found a variety of uses including use as a titrant for acids in organic solvents and as a supporting electrolyte in polarography. Aqueous solutions of quaternary ammonium hydroxides, particularly TMAH solutions, have been used extensively as a developer for photoresists in printed circuit board and microelectronic chip fabrication. Use of quaternary ammonium hydroxides in the electronics area requires that there be no residue following the normal post-bake period. In electronic applications, it is desirable that the aqueous solutions of quaternary ammonium hydroxides should be essentially free from metal ions such as sodium, potassium, zinc and calcium; anions such as halides, nitrates, nitrites, carbonates, carboxylates, sulfates and neutral organic species such as methanol, amines, etc. Particularly in recent years, there has been an increasing demand for quaternary ammonium hydroxides having a high purity.
Quaternary ammonium hydroxides such as TMAH and TEAH have been produced by various techniques. Generally, the quaternary ammonium hydroxides are manufactured by electrolyzing a salt of a quaternary ammonium compound in an electrolysis cell containing a diaphragm formed of a cation-exchange membrane. The quaternary ammonium salts used in such preparations include halide salts, carboxylate salts, carbonate salts and sulfate salts. When halide salts are used in the manufacture of quaternary ammonium hydroxide, it has been discovered that the quaternary ammonium hydroxide solutions formed by this method generally contain significant amounts of halogen (ionic and latent), generally in concentrations from about 30 ppm up to about 100 ppm at 25% quaternary ammonium hydroxide (e.g., TMAH). The term "latent halide" is used throughout this specification and claims to refer to non-ionic halogen which is present in the aqueous quaternary ammonium hydroxide solutions, and which is capable of forming halide ions under certain conditions such as heating.
Among the prior art patents which describe the preparation of quaternary ammonium hydroxides by electrolyzing a salt of a quaternary ammonium compound are U.S. Pat. Nos. 4,572,769 (Shimizu); 4,521,285 (DeWitt et at); 4,425,202 (Sullivan); and 4,394,226 (Wade et at). U.S. Pat. No. 4,572,769 describes the use of formate salts to form the quaternary ammonium hydroxides, and this patent suggests that some of the problems of using quaternary ammonium halides are minimized by use of the formate salt. The formate salts are prepared by the reaction of a trialkyl amine with methyl formate in either methanol or ethanol as solvent. U.S. Pat. No. 4,521,285 describes an electrolytic process for removing the anion from quaternary organic salts. The process uses a cell comprising four compartments containing two cation exchange membranes and one anion exchange membrane. U.S. Pat. No. 4,425,202 describes a process for making choline base by electrolysis of choline chloride in an electrolytic cell. Color stabilization of choline base is effected through concentration control and/or the addition of a sulfite prior to electrolytic manufacture of the choline base. U.S. Pat. No. 4,394,226 describes production of quaternary ammonium hydroxides in electrolytic cells using cationic membranes which have been treated with a mineral acid prior to use in the electrolysis.
U.S. Pat. No. 4,714,530 (Hale et al) describes an electrolytic process for preparing high purity quaternary ammonium hydroxides which utilizes a cell containing a catholyte compartment and an anolyte compartment separated by a cation-exchange membrane. The process comprises charging an aqueous solution of a quaternary ammonium hydroxide to the anolyte compartment, adding water to the catholyte compartment, and passing a direct current through the electrolysis cell to produce a higher purity quaternary ammonium hydroxide in the catholyte compartment which is subsequently recovered. The '530 patent also describes an improvement which comprises heating the quaternary ammonium hydroxide at an elevated temperature prior to charging the hydroxide to the anolyte compartment of the electrolytic cell.
U.S. Pat. No. 4,938,854 (Sharifian et at) also describes an electrolytic process for purifying quaternary ammonium hydroxides by lowering the latent halide content. The electrolytic cell may be divided into an anolyte compartment and a catholyte compartment by a divider which may be an anion or cation selective membrane. The cathode in the catholyte compartment comprises zinc, cadmium, tin, lead, copper or titanium, or alloys thereof, mercury or mercury amalgam.
Japanese Kokai Patent No. 60-131985 (1985) (Takahashi et al) describes a method of manufacturing a high purity quaternary ammonium hydroxide in an electrolysis cell which is divided into an anode chamber and a cathode chamber by a cation exchange membrane. A quaternary ammonium hydroxide solution containing impurities is charged to the anode chamber and a direct current is supplied between two electrodes after water has been charged to the cathode chamber. Purified quaternary ammonium hydroxide is obtained from the cathode chamber. The purified quaternary ammonium hydroxide contains reduced amounts of alkali metals, alkaline earth metals, anions, etc.
Japanese Kokai Patent No. 60-131986 (1985) (Takahashi et at) describes a method for manufacturing a high purity quaternary ammonium hydroxide. The method described in this patent utilizes an electrolysis cell which has been compartmentalized into an anode chamber, a cathode chamber, and at least one intermediate chamber with at least two cation exchange membranes. An aqueous solution containing a quaternary ammonium salt is charged to the anode chamber, water is charged to the cathode chamber, and an aqueous hydroxide solution corresponding to the quaternary ammonium salt charged into the anode chamber is charged into the intermediate chamber. Upon application of a direct current, a quaternary ammonium hydroxide is formed in the cathode chamber and recovered.